A slide or gate valve assembly generally includes a housing defining a flow path extending between an inlet and an outlet. The slide valve assembly can, for example, be connected between a process chamber and a vacuum pump to form a portion of a high purity gas delivery system, such as those used in semiconductor manufacturing or other thin film coating processes performed at very low pressures (high vacuums), e.g., a pressure on the order of one torr or less. In such a case, a flange of the inlet of the valve assembly is secured to the process chamber while a flange of the outlet of the valve assembly is secured to the vacuum pump.
The slide valve assembly includes a slide plate that laterally moves, with respect to an axis of the flow path of the valve assembly, between open and closed positions. In a pendulum-type slide valve assembly, the slide plate is connected to a rotatably shaft by a pivot arm. In a completely opened position the slide plate is moved out of the flow path of the housing so that fluid can freely enter and exit the flow path, while in a closed position the slide plate is moved into close contact with a valve seat or annular surface surrounding the outlet of the valve assembly, so that conductance through the flow path is limited. The movement of the slide plate usually requires rotational (i.e., pivotal or lateral) movement between the completely opened position (i.e., first opened position) and an intermediate position (i.e., second opened position), and then at least some longitudinal (i.e., translational, linear or axial) movement from the intermediate position to the closed position where the slide plate is in close contact with the valve seat of the outlet.
U.S. Pat. No. 6,089,537 to Olmsted, which is assigned to the assignee of the present disclosure and incorporated herein by reference, discloses a pendulum valve assembly that uses a simple rotating cam mechanism that precisely controls the rotational and longitudinal movement of the slide plate between a completely opened position and a completely closed position. The cam mechanism also includes a spring which acts to bias the slide plate away from the valve seat of the outlet.
Some existing pendulum valves further include a seal ring to provide a complete seal (isolation) when the seal ring is independently actuated against the slide plate. U.S. Pat. No. 5,577,707 to Brida, for example, discloses a pendulum-type slide valve including a seal ring that is movable towards and away from the slide plate, when the slide plate is pivoted to a closed position. The seal ring is biased, using a spring or compressed air, against the slide plate when the slide plate is in its closed position, to tightly close the slide plate against the valve seat and to form a seal between the slide plate and the seal ring.
The seal ring typically provides an isolation function for the valve assembly and is not used to control conductance. Normally the seal ring is held stationary and the slide plate is moved to control conductance. However, in co-pending U.S. patent application Ser. No. 10/673,989, filed on Sep. 29, 2003, which is assigned to the assignee of the present disclosure and incorporated herein by reference, a pendulum valve assembly is disclosed that has a seal ring adapted to provide improved conductance control during the initial opening of the valve.
Operation of the valve assembly between the process chamber and the vacuum pump usually includes three stages: isolation, pump-down, and throttling. During isolation, the seal ring provides an isolation function and the pressure differential between the inlet and the outlet of the valve assembly is large. During pump-down, the seal ring is moved away from the slide plate and conductance gradually increases between the slide plate and the valve seat of the outlet, and the pressure differential between the inlet and the outlet of the valve assembly decreases. During throttling, conductance between the slide plate and the valve seat of the outlet is maintained at a constant, predetermined level and the pressure differential between the inlet and the outlet of the valve assembly is relatively low but remains constant. The throttling procedure is used to maintain a desired low pressure (i.e., processing pressure) within the attached process chamber during the processing of semiconductor devices.
One drawback for some existing gate or pendulum valves is that, when a high pressure differential exists between the chamber side and the pump side of the valve, a large force is needed to initially move the slide plate and open the valve. Such a large force requires a large drive mechanism, which adds to the size and cost of the valve assembly. Alternatively, a separate bypass valve may be connected to the chamber side of the pendulum valve and operated to lower the pressure in the chamber side of the pendulum valve prior to opening the pendulum valve, such that a large drive mechanism is not needed to initially move the slide plate. Again, however, a separate bypass valve adds to the size and cost of the pendulum valve assembly.
If a large drive mechanism or a separate bypass valve is not used, then the pump-down procedure may require a long period for conductance to develop between the slide plate and the valve seat of the outlet, and for the pressure differential to drop to a desired level. Long pump-down periods, however, are unwanted in the processing of semiconductor devices since such long periods ultimately reduce the output of processed semiconductor devices.
What is still desired is a new and improve valve assembly. Preferably, the new and improved valve assembly will be adapted such that a high pressure differential between the chamber side (i.e., inlet of valve assembly) and the pump side (i.e., outlet) of the valve assembly is automatically reduced prior to opening of the slide plate, so that a large drive mechanism or a separate bypass valve is not needed to initially move the slide plate and open the valve during a pump-down procedure. In addition, the automatic reduction of the high pressure differential preferably will provide shorter pump-down periods during the processing of semiconductor devices.